In the fields of both monochromatic and color ink jet printing using, for example, thermal ink jet printers of the type operative with disposable TIJ pens, various approaches have been taken to ensure that these pens were constructed to have a reasonably large ink storage capacity in order to give these pens a commercially acceptable lifetime. It has been a common practice to construct these pens so that a thin film resistor (TFR) type of printhead device could be mounted on or adjacent to one surface of the pen body housing and an ink storage compartment arranged within the housing and in ink flow communication with the thin film resistor printhead. However, in addition to providing an adequate ink storage capacity for these disposable ink jet pens, it is also a requirement that a controlled negative pressure or back pressure be maintained at the output ink ejection orifice plate of the thin film resistor printhead. This is done in order to ensure that ink does not drool or drip from the printhead with insufficient back pressure or does not deprime by the use of too much back pressure generated within the ink storage compartment.
In U.S. Pat. No. 4,500,895 issued to Roy T. Buck et al and assigned to the present assignee, there is disclosed a disposable thermal ink jet pen which utilizes a collapsible bladder as the ink storage compartment for the pen. This bladder has been constructed to collapse gradually during ink depletion therein, and it operates to provide a range of relatively constant back pressures as the pen is depleted from full to empty. However, as a result of the non-linearity in the back pressure versus ink depletion characteristic of the pen, these pens are hard to scale up to larger pen body constructions in such a manner that the back pressure maintained by the bladder is substantially constant and closely controlled.
Another approach to maintaining and improving the control over the necessary constant back pressure at the thin film resistor printhead of a thermal ink jet pen has been to use a reticulated polyurethane foam in either the black or color storage compartments of the pen. This type of foam material has served quite satisfactorily to not only maintain the necessary constant back pressure in the pen, but also to prevent the ink from sloshing around within the pen body housing during its rapid back and forth movement in a pen carriage member of a thermal ink jet printer. One such approach using a foam material as the ink storage medium is disclosed and claimed in U.S. Pat. No. 4,771,295 issued to Jeffrey P. Baker et al and also assigned to the present assignee.
In order to provide yet another approach to maintaining good control over the back pressure at the printhead of a thermal ink jet pen while simultaneously increasing its ink storage capacity, we have discovered and developed a novel alternative pen body construction which uses, among other things, a thin hydrophobic membrane which is positioned between an ink storage reservoir and an air space within an ink receiving compartment of the pen. A thin film resistor printhead is mounted adjacent to an output surface of the ink receiving compartment and operates to draw ink from the main ink reservoir into the ink receiving compartment when the differential pressure across the thin hydrophobic membrane exceeds the inherent bubble pressure of the membrane. This novel pen body construction is disclosed and claimed in our co-pending application Ser. No. 07/414,893 of Alfred I. Pan and C. S. Chan entitled "Ink Delivery System For Printers", also assigned to the present assignee and incorporated herein by reference.